Current trends in the design of automotive engines are associated with higher operating temperatures as the efficiency of the engines increases and these higher operating temperatures require successively higher quality lubricants. One of the requirements is for higher viscosity indices (V.I.) in order to reduce the effects of the higher operating temperatures on the viscosity of the engine lubricants. High V.I. values have conventionally been attained by the use of V.I. improvers such as polyacrylates, but there is a limit to the degree of improvement which may be effected in this way. In addition, V.I. improvers tend to undergo degradation under the effects of high temperatures and high shear rates encountered in the engine, the more stressing conditions encountered in high efficiency engines result in even faster degradation of oils which employ significant amounts of V.I. improvers. Thus, there is a continuing need for automotive lubricants which are based on fluids of high viscosity index and which are stable to the high temperature, high shear rate conditions encountered in modern engines.
Synthetic lubricants produced by the polymerization of alpha olefins in the presence of certain catalysts have been shown to possess excellent V.I. values. Oligomers of 1-alkenes from C.sub.6 to C.sub.20, especially 1-decene, have been converted to commercially useful synthetic lubricants by oligomerization with Lewis acid catalysts such as boron trifluoride. Examples of such processes are described, for example, in U.S. Pat. No. 4,395,578 which describes the oligomerization of alpha-olefins using boron trifluoride catalyst; the alpha-olefins are produced by ethylene polymerization or wax pyrolysis. Viscosity indices up to 112 are said to be achieved. Similarly, U.S. Pat. No. 4,420,646 describes a process for the production of synthetic lubricants by the oligomerization of alpha-olefins produced from wax pyrolysis. Boron trifluoride catalyst providing viscosity indices of about 130 are reported.
These synthetic products are, however, expensive to produce by conventional synthetic procedures and usually require expensive starting materials. There is therefore a need for the production of high V.I. lubricants from mineral oil stocks which may be produced by techniques comparable to those presently employed in petroleum refineries.
In prior application Ser. No. 07/571,345, we have described a process for making synthetic lubricants of high viscosity index from slack wax by a process of thermal cracking followed by oligomerization in the presence of an aluminum trichloride catalyst. The use of the cheap slack wax feed is notable in giving oligomerization products which result in a higher product VI and viscosity; the use of the aluminum trichloride catalyst also results in a higher yield with the wax-derived olefins.
Another approach to the production of high quality synthetic lubricants is described in U.S. Pat. Nos. 4,827,064 and 4,827,073. These patents describe the production of synthetic lubricants from alpha-olefins such as 1-decene wiuth catalysts made up of an oxide of a metal of Group VI of the Periodic Table in a reduced valence state, preferably a chromium catalyst produced by the reduction in carbon monoxide of a silica-supported chromium oxide. The olefin oligomers produced in this way exhibit extraordinarily high VI values, typically as high as about 200. The process is also favorable in that it uses a heterogeneous catalyst which can be used in a simple fixed-bed operation, permitting ready separation of the oligomerization products; the catalyst may also be readily regenerated by an oxidation-reduction process, as described in U.S. Pat. No. 4,996,384. Reference is made to U.S. Pat. Nos. 4,827,064; 4,827,073 and 4,996,384 for a descrition of the reduced metal oxide catalysts used in the production of the HVI-PAO synthetic lubricants, as well as for a descrition of the methods by which the lubricants are made and of their properties.